Hi.
While experimenting with the implicit parameter
extension to Haskell 98, implemented in GHC 4.08.1
and latest Hugs, I came accross a difference among
those implementations regarding overloading functions
with implicit parameters.
As a test consider the program
------------------------- cut here
module Main where
class C a where
f :: (?env :: Integer) => a -> Integer
instance C Integer where
f x = ?env + x
main = putStrLn (show (f (45::Integer) with ?env = 100))
------------------------- cut here
Hugs accepts this program and outputs 145, as expected.
But GHC 4.08.1 refuses to compile it, emitting the
message
$ ghc -fglasgow-exts Test1.hs -o test1
Test1.hs:7:
Unbound implicit parameter `env_rJX :: Integer'
arising from use of `env_rJX' at Test1.hs:7
In the first argument of `+', namely `env_rJX'
In the right-hand side of an equation for `f': env_rJX + x
Compilation had errors
Would anybody comment on what is going on with GHC?
I am willing to use implicit parameters in the
software I am developing, but I have the need
to overload functions with implicit parameters.
While Hugs is good for development, its performance
may rule it out when the final product is ready.
So I will need a good Haskell compiler to compile
my system.
Any comments?
Romildo
--
Prof. José Romildo Malaquias <romildo@iceb.ufop.br>
Departamento de Computação
Universidade Federal de Ouro Preto
Brasil